Enhancing the Reversibility of Mg/S Battery Chemistry through Li+ Mediation

Transition metal dichalcogenide-based materials for rechargeable aluminum-ion batteries: A mini-review

Rechargeable aluminum-ion batteries (AIBs) have emerged as a promising candidate for energy storage applications and have been extensively investigated over the past few years. Due to their high theoretical capacity, nature of abundance, and high safety, AIBs can be considered an alternative to lithium-ion batteries. However, the electrochemical performance of AIBs for large-scale applications is still limited due to the poor selection of cathode materials. Transition metal dichalcogenides (TMDs) have been regarded as appropriate cathode materials for AIBs due to their wide layer spacing, large surface area, and distinct physiochemical characteristics. This mini-review provides a succinct summary of recent research progress on TMD-based cathode materials in non-aqueous AIBs. The latest developments in the benefits of utilizing 3D-printed electrodes for AIBs are also explored.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Polysulfides in Magnesium-Sulfur Batteries

Mg-S batteries hold great promise as a potential alternative to Li-based technologies. Their further development hinges on solving a few key challenges, including the lower capacity and poorer cycling performance when compared to Li counterparts. At the heart of the issues is the lack of knowledge on polysulfide chemical behaviors in the Mg-S battery environment. In this Review, a comprehensive overview of the current understanding of polysulfide behaviors in Mg-S batteries is provided. First, a systematic summary of experimental and computational techniques for polysulfide characterization is provided. Next, conversion pathways for Mg polysulfide species within the battery environment are discussed, highlighting the important role of polysulfide solubility in determining reaction kinetics and overall battery performance. The focus then shifts to the negative effects of polysulfide shuttling on Mg-S batteries. The authors outline various strategies for achieving an optimal balance between polysulfide solubility and shuttling, including the use of electrolyte additives, polysulfide-trapping materials, and dual-functional catalysts. Based on the current understanding, the directions for further advancing knowledge of Mg polysulfide chemistry are identified, emphasizing the integration of experiment with computation as a powerful approach to accelerate the development of Mg-S battery technology.

A High-Energy Four-Electron Zinc Battery Enabled by Evoking Full Electrochemical Activity in Copper Sulfide Electrode

The growing global demand for sustainable and cost-effective energy storage solutions has driven the rapid development of zinc batteries. Despite significant progress in recent years, enhancing the energy density of zinc batteries remains a crucial research focus. One prevalent strategy involves the development of high-capacity and/or high-voltage cathode materials. CuS, a commonly used electrode material, exhibits a two-electron transfer mechanism; however, the reduced sulfion lacks electrochemical activity and thereby limits its discharge capacity and redox potential. In this study, we activate a CuS cathode to form a high-valence Cu2+&S compound using a deep-eutectic-solvent (DES)-based electrolyte. The presence of Cl- in the DES-based electrolyte is crucial to the reversibility of the redox chemistry, and the liquid-phase-involved electrochemical process facilitates redox kinetics. A four-electron transfer pathway involving five reaction steps is identified for the CuS electrode, which unleashes the full electrochemical activity of the S element. Consequently, the full cell delivers a large discharge capacity of ∼800 mAh g-1 at 0.2 A g-1 and yields a high discharge plateau starting at 1.58 V, contributing to energy densities of up to 650 Wh kg-1 (based on CuS). This work offers a promising approach to developing high-energy zinc batteries.

Nanosynthesis and Characterization of Cu1.8Se0.6S0.4 as a Potential Cathode for Magnesium Battery Applications

Copper selenide (Cu-Se) and copper sulfide (Cu-S) are promising cathodes for magnesium-ion batteries. However, the low electronic conductivity of Cu-Se system results in a poor rate capability and unsatisfactory cycling performance. Mg-ion batteries based on the Cu-S cathode exhibited large kinetic barriers during the recharging process owing to the presence of polysulfide species. This work attempts to circumvent this dilemma by doping Cu1.8Se by sulfur, which replaces the selenium in the CuSe lattice to form Cu1.8Se0.6S0.4 nanocrystalline powder. The presence of sulfur will increase the electronic conductivity, and the presence of selenium will mitigate the effect of polysulfide species that hinder the kinetics of Mg2+. Herein, a Cu1.8Se0.6S0.4 nanocrystalline powder was synthesized by the solid-state reaction, yielding a highly pure and stoichiometric powder. The crystallographic structure of the nanopowder and the conversion-type storage mechanism have been attested via ex situ X-ray diffraction and energy-dispersive X-ray analysis. The nanocrystalline feature of Cu1.8Se0.6S0.4 was demonstrated by high-resolution transmission electron microscopy. An apparent surface morphology change during the charging/discharging process has been visualized by a field emission scanning electron microscope. Diffuse reflectance spectroscopy has discussed the variation of the band gap during charging and discharging. The full Mg/Cu1.8Se0.6S0.4 cells presented an initial discharge capacity of 387.99 mAh g-1 at a current density of 0.02 mA cm-2; moreover, they show moderate diffusion kinetics with D Mg 2 + ≈ 10-15 cm-2 s-1.

First-principles investigation on multi-magnesium sulfide and magnesium sulfide clusters in magnesium-sulfide batteries

Because of the abundance of magnesium and sulfur and their low cost, the development of magnesium sulfur batteries is very promising. In particular, the battery performance of nanoscale (MgS)_n clusters is much better than that of bulk sized MgS. However, the structures, stability, and properties of Mg_xS_y and (MgS)_n clusters, which are very important to improve the performance of Mg-S batteries, are still unexplored. Herein, the most stable structures of Mg_xS_y (x = 1-8, y = 1-8) and (MgS)_n (n = 1-10) are reliably determined using the structure search method and density functional theory to calculate. According to calculation results, MgS₃ and Mg₆S₈ may not exist in the actual charging and discharging products of magnesium sulfide batteries. The (MgS)_n (n ≥ 5) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the MgS monomer, each sulfur atom in the clusters is coordinated with more magnesium atoms, thus lengthening the Mg-S bond length and decreasing the Mg-S bond activation energy. Notably, with the increase of dielectric constant of electrolyte solvent, compared to the DME (ε = 7.2), THF (ε = 7.6) and C₂H₄Cl₂ (ε = 10.0), Mg_xS_y and (MgS)_n clusters are most stable in the environment of C₃H₆O (ε = 20.7). It can delay the transformation of magnesium polysulfide to the final product MgS, which is conducive to improving the performance of Mg-S batteries. The predicted characteristic peaks of infrared and Raman spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (MgS)_n clusters and improving the performance of Mg-S batteries.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

Toward High-Performance Mg-S Batteries via a Copper Phosphide Modified Separator

Magnesium-sulfur (Mg-S) batteries are emerging as a promising alternative to lithium-ion batteries, due to their high energy density and low cost. Unfortunately, current Mg-S batteries typically suffer from the shuttle effect that originates from the dissolution of magnesium polysulfide intermediates, leading to several issues such as rapid capacity fading, large overcharge, severe self-discharge, and potential safety concern. To address these issues, here we harness a copper phosphide (Cu3P) modified separator to realize the adsorption of magnesium polysulfides and catalyzation of the conversion reaction of S and Mg2+ toward stable cycling of Mg-S cells. The bifunctional layer with Cu3P confined in a carbon matrix is coated on a commercial polypropylene membrane to form a porous membrane with high electrolyte wettability and good thermal stability. Density functional theory (DFT) calculations, polysulfide permeability tests, and post-mortem analysis reveal that the catalytic layer can adsorb polysulfides, effectively restraining the shuttle effect and facilitating the reversibility of the Mg-S cells. As a result, the Mg-S cells can achieve a high specific capacity, fast rates (449 mAh g-1 at 0.1 C and 249 mAh g-1 at 1.0 C), and a long cycle life (up to 500 cycles at 0.5 C) and operate even at elevated temperatures.

Organic Electrolyte Design for Rechargeable Batteries: From Lithium to Magnesium

Rechargeable magnesium batteries (RMBs) have been considered as one of the most viable battery chemistries amongst the "post" lithium-ion battery (LIB) technologies owing to their high volumetric capacity and the natural abundance of their key elements. The fundamental properties of Mg-ion conducting electrolytes are of essence to regulate the overall performance of RMBs. In this Review, the basic electrochemistry of Mg-ion conducting electrolytes batteries is discussed and compared to that of the Li-ion conducting electrolytes, and a comprehensive overview of the development of different Mg-ion conducting electrolytes is provided. In addition, the remaining challenges and possible solutions for future research are intensively discussed. The present work is expected to give an impetus to inspire the discovery of key electrolytes and thereby improve the electrochemical performances of RMBs and other related emerging battery technologies.

Long-Cycle-Life Calcium Battery with a High-Capacity Conversion Cathode Enabled by a Ca2+/Li+ Hybrid Electrolyte

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (-2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca²⁺ plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS₂ as a conversion cathode material and combined it with a Li⁺/Ca²⁺ hybrid electrolyte for Ca batteries. We demonstrate that Li⁺ ions ensured reversible Ca²⁺ plating/stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS₂, offering high discharge capacity. As a result, the Ca/FeS₂ cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g^-1 over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.

A Magnesium/Lithium Hybrid-Ion Battery with Modified All-Phenyl-Complex-Based Electrolyte Displaying Ultralong Cycle Life and Ultrahigh Energy Density

Magnesium/lithium hybrid-ion batteries (MLHBs) combine the advantages of high safety and fast ionic kinetics, which enable them to be promising emerging energy-storage systems. Here, a high-performance MLHB using a modified all-phenyl complex with a lithium bis(trifluoromethanesulfonyl)imide electrolyte and a NiCo₂S₄ cathode on a copper current collector is developed. A reversible conversion involving a copper collector with NiCo₂S₄ efficiently avoids the electrolyte dissociation and diffusion difficulties of Mg²⁺ ions, enabling low polarization and fast redox, which is verified by X-ray absorption near edge structure analysis. Such combination affords the best MLHB among all those ever reported, with a reversible capacity of 204.7 mAh g^-1 after 2600 cycles at 2.0 A g^-1, and delivers an ultrahigh full electrode-basis energy density of 708 Wh kg^-1. The developed MLHB also achieves good rate performance and temperature tolerance at -10 and 50 °C with a low electrolyte consumption. The hybrid-ion battery system presented here could inspire a broad set of engineering potentials for high-safety battery technologies and beyond.

Origin of Excellent Charge Storage Properties of Defective Tin Disulphide in Magnesium/Lithium-Ion Hybrid Batteries

Lithium-ion batteries (LIBs) are excellent electrochemical energy sources, albeit with existing challenges, including high costs and safety concerns. Magnesium-ion batteries (MIBs) are one of the potential alternatives. However, the performance of MIBs is poor due to their sluggish solid-state Mg²⁺ diffusion kinetics and severe electrode polarizability. Rechargeable magnesium-ion/lithium-ion (Mg²⁺/Li⁺) hybrid batteries (MLHBs) with Mg²⁺ and Li⁺ as the charge carriers create a synergy between LIBs and MIBs with significantly improved charge transport kinetics and reliable safety features. However, MLHBs are yet to reach a reasonable electrochemical performance as expected. This work reports a composite electrode material with highly defective two-dimensional (2D) tin sulphide nanosheets (SnS_x) encapsulated in three-dimensional (3D) holey graphene foams (HGF) (SnS_x/HGF), which exhibits a specific capacity as high as 600 mAh g^-1 at 50 mA g^-1 and a compelling specific energy density of ~ 330 Wh kg^-1. The excellent electrochemical performance surpasses previously reported hybrid battery systems based on intercalation-type cathode materials under comparable conditions. The role played by the defects in the SnS_x/HGF composite is studied to understand the origin of the observed excellent electrochemical performance. It is found that it is closely related to the defect structure in SnS_x, which offers percolation pathways for efficient ion transport and increased internal surface area assessable to the charge carriers. The defective sites also absorb structural stress caused by Mg²⁺ and Li⁺ insertion. This work is an important step towards realizing high-capacity cathode materials with fast charge transport kinetics for hybrid batteries.

A High Potential Polyanion Cathode Material for Rechargeable Mg-Ion Batteries

The development of Mg-ion batteries is hindered by the lack of cathode materials that allow facile and reversible Mg²⁺ intercalation at high potential. Herein, the authors present a polyanion cathode material of K₂ (VO)₂ (HPO₄ )₂ (C₂ O₄ )⋅4.5H₂ O (KVPCH) for Mg-ion batteries. The inductive effect of polyanions ensures the high redox potential of the vanadium centers. In addition, the material contains structural water located between the layers. It helps with Mg²⁺ desolvation at the electrode-electrolyte interface and facilitates its diffusion in the structure, as confirmed by experimental analysis and theoretical calculations. Thanks to those factors, the KVPCH electrode presents excellent Mg storage capability at room temperature. It delivers 121 mAh g^-1 capacity at 1C with a high average discharge potential of 0.16 V versus Ag/Ag⁺ (3.2 V vs Mg/Mg²⁺ ). A capacity retention of 87% is realized after 1500 cycles at 5C. A rocking-chair Mg-ion full cell is also demonstrated with the KVPCH cathode and a MoO_x anode, which achieves 46 mAh g^-1 capacity (based on the total active material mass of two electrodes). This work would bring effective paths for the design of cathode materials for Mg-ion batteries.

Recent Advancements in Chalcogenides for Electrochemical Energy Storage Applications

Energy storage has become increasingly important as a study area in recent decades. A growing number of academics are focusing their attention on developing and researching innovative materials for use in energy storage systems to promote sustainable development goals. This is due to the finite supply of traditional energy sources, such as oil, coal, and natural gas, and escalating regional tensions. Because of these issues, sustainable renewable energy sources have been touted as an alternative to nonrenewable fuels. Deployment of renewable energy sources requires efficient and reliable energy storage devices due to their intermittent nature. High-performance electrochemical energy storage technologies with high power and energy densities are heralded to be the next-generation storage devices. Transition metal chalcogenides (TMCs) have sparked interest among electrode materials because of their intriguing electrochemical properties. Researchers have revealed a variety of modifications to improve their electrochemical performance in energy storage. However, a stronger link between the type of change and the resulting electrochemical performance is still desired. This review examines the synthesis of chalcogenides for electrochemical energy storage devices, their limitations, and the importance of the modification method, followed by a detailed discussion of several modification procedures and how they have helped to improve their electrochemical performance. We also discussed chalcogenides and their composites in batteries and supercapacitors applications. Furthermore, this review discusses the subject’s current challenges as well as potential future opportunities.

Dual Role of Mo6 S8 in Polysulfide Conversion and Shuttle for Mg-S Batteries

Magnesium-Sulfur batteries are one of most appealing options among the post-lithium battery systems due to its potentially high energy density, safe and sustainable electrode materials. The major practical challenges are originated from the soluble magnesium polysulfide intermediates and their shuttling between the electrodes, which cause high overpotentials, low sulfur utilization, and poor Coulombic efficiency. Herein, a functional Mo₆ S₈ modified separator is designed to effectively address these issues. Both the experimental results and density functional theory calculations show that the electrochemically active Mo₆ S₈ layer has a superior adsorption capability of polysulfides and simultaneously acts as a mediator to accelerate the polysulfide conversion kinetics. Remarkably, the magnesium-sulfur cell assembled with the functional separator delivers a high specific energy density (942.9 mA h g^-1 in the 1st cycle) and can be cycled at 0.2 C for 200 cycles with a Coulombic efficiency of 96%. This work demonstrates a new design concept toward high-performance metal-sulfur batteries.

Life-Related Hazards of Materials Applied to Mg–S Batteries

Nowadays, rechargeable batteries utilizing an S cathode together with an Mg anode are under substantial interest and development. The review is made from the point of view of materials engaged during the development of the Mg–S batteries, their sulfur cathodes, magnesium anodes, electrolyte systems, current collectors, and separators. Simultaneously, various hazards related to the use of such materials are discussed. It was found that the most numerous groups of hazards are posed by the material groups of cathodes and electrolytes. Such hazards vary widely in type and degree of danger and are related to human bodies, aquatic life, flammability of materials, or the release of flammable or toxic gases by the latter.

Electrocatalytic Activity of Polyaniline in Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur (Mg-S) batteries offer the potential for inexpensive energy storage alternatives to other metal-ion batteries for the grid scale and household applications. Despite all economic and resource advantages, Mg-S battery chemistry suffers from a complicated reaction mechanism and extremely slow reaction kinetics. To improve the kinetics, we improvise a new electrode architecture where a conductive polymer is used along with a carbon network. This report will bring an important insight of electrocatalytic activity of polyaniline, on the basis of free-radical coupling and is a completely new concept in Mg-S battery chemistry. By the combined electron spin resonance spectroscopy, X-ray photoelectron spectroscopy, and fluorescence lifetime measurements, we perceived that the polyaniline anchors the S₃^•- species from the electrolyte/catholyte through a free-radical-coupling process and thus promotes the reduction to end-discharged products, via a chemical adduct. The concept of free-radical catalysis in Mg/S batteries will open a new knowledge to enhance the active material utilization in the Mg-S batteries.

Probing the Polysulfide Confinement in Two Different Sulfur Hosts for a Mg|S Battery Employing Operando Raman and Ex-Situ UV-Visible Spectroscopy

We study here the Mg-polysulfide confinement inside two structurally different model porous materials, viz., toray carbon paper (TC) and multiwalled carbon nanotubes (CNT), using operando Raman and postcycling ex-situ UV-vis spectroscopy. Sulfur encapsulated inside CNT (CNT-S) and TC (TC-S) serves as S-cathodes in a rechargeable room temperature Mg|S battery. Operando Raman spectroscopy indicates the presence of higher-order Mg-polysulfides at the CNT cathode. This is due to the combination of their entrapment inside CNT and also possibly to their localization in the liquid electrolyte in the vicinity of CNT-S. This finding is directly correlated to the ex-situ UV-vis spectroscopy, which shows a lesser degree of Mg-polysulfide dissolution into the electrolyte solution. In comparison, TC-S, where sulfur is encapsulated within the open matrix formed by the nanofiber network of the carbon paper, displays poorer polysulfide confinement. The distinct differences in their abilities to confine the Mg-polysulfides are corroborated by battery performance. In the current density range (0.05-1) C, the battery with CNT-S displays much higher specific capacities, being nearly two times that of TC-S at 1 C.

Hierarchical 3D Cuprous Sulfide Nanoporous Cluster Arrays Self-Assembled on Copper Foam as a Binder-Free Cathode for Hybrid Magnesium-Based Batteries

On account of easy accessibility, high theoretical volumetric capacity and dendrite-free magnesium (Mg) anode, Mg battery has a great promise to be next generation rechargeable batteries, yet still remains a challenging task in acquiring fast Mg²⁺ kinetics and effective cathode materials. Herein, hierarchical 3D cuprous sulfide porous nanosheet decorated nanowire cluster arrays with robust adhesion on copper foam (Cu₂ S HP/CF), which is employed as a binder-free conversion cathode material for magnesium/lithium hybrid battery, delivering impressively initial and reversible specific capacity of 383 and 311 mAh g^-1 at 100 mA g^-1 , respectively, which are obviously outperformed corresponding powder cathode in a traditional method by using polymer binder, is reported. Intriguingly, benefiting from the hierarchical nanoporous array architecture and self-assembly feature, Cu₂ S HP/CF cathode shows a remarkable cycling stability with a high capacity of 129 mAh g^-1 at 300 mA g^-1 over 500 cycles. This work not only highlights a guide for designing hierarchical nanoporous materials derived from metal-organic frameworks, but also provides a novel strategy of in situ formation to fabricate binder-free cathodes.

Current Design Strategies for Rechargeable Magnesium-Based Batteries

As a next-generation electrochemical energy storage technology, rechargeable magnesium (Mg)-based batteries have attracted wide attention because they possess a high volumetric energy density, low safety concern, and abundant sources in the earth's crust. While a few reviews have summarized and discussed the advances in both cathode and anode materials, a comprehensive and profound review focusing on the material design strategies that are both representative of and peculiar to the performance improvement of rechargeable Mg-based batteries is rare. In this mini-review, all nine of the material design strategies and approaches to improve Mg-ion storage properties of cathode materials have been comprehensively examined from both internal and external aspects. Material design concepts are especially highlighted, focusing on designing "soft" anion-based materials, intercalating solvated or complex ions, expanding the interlayer of layered cathode materials, doping heteroatoms into crystal lattice, size tailoring, designing metastable-phase materials, and developing organic materials. To achieve a better anode, strategies based on the artificial interlayer design, efficient electrolyte screening, and alternative anodes exploration are also accumulated and analyzed. The strategy advances toward Mg-S and Mg-Se batteries are summarized. The advantages and disadvantages of all-collected material design strategies and approaches are critically discussed from practical application perspectives. This mini-review is expected to provide a clear research clue on how to rationally improve the reliability and feasibility of rechargeable Mg-based batteries and give some insights for the future research of Mg-based batteries as well as other multivalent-ion battery chemistries.

A Pyrite Iron Disulfide Cathode with a Copper Current Collector for High-Energy Reversible Magnesium-Ion Storage

Owing to its low cost, high theoretical capacity, and environmentally friendly characteristics, pyrite FeS₂ demonstrates promise as a cathode material for high-energy metal-anode-based rechargeable batteries. When it is used in a rechargeable magnesium battery (RMB), the electrode couple exhibits an extremely low theoretical volume change upon full discharge. However, its electrochemical Mg-ion storage is considerably hindered by slow reaction kinetics. In this study, a high-performance FeS₂ cathode for RMBs using a copper current collector is reported, which is involved in cathode reactions via a reversible redox process between copper and cuprous sulfide. This phase transformation with the formation of copper nanowires during discharge activates the redox reactions of FeS₂ via a two-step and four-electron Mg-ion transfer that dominates the cathode reactions. As a result, the as-prepared FeS₂ nanomaterial cathode delivers a significantly enhanced reversible capacity of 679 mAh g^-1 at 50 mA g^-1 . The corresponding energy density of 714 Wh kg^-1 is superior to those of all previously reported metal chalcogenide cathodes in RMBs or hybrid batteries using a Mg metal anode. Notably, the as-assembled FeS₂ -Mg battery can operate over 1000 cycles with a good capacity retention at 400 mA g^-1 .

A Chlorine-Free Electrolyte Based on Non-nucleophilic Magnesium Bis(diisopropyl)amide and Ionic Liquid for Rechargeable Magnesium Batteries

The electrolyte based on magnesium bis(diisopropyl)amide (MBA), a low-cost and non-nucleophilic organic magnesium salt, is proposed to be an admirable alternative for rechargeable magnesium batteries but suffers from limited ionic conductivity and an inferior electrochemical window in the commonly used ether solvents. In this work, the 1-butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP₁₄TFSI) ionic liquid as the cosolvent of tetrahydrofuran (THF) in chlorine-free MBA-based electrolytes has been first demonstrated to remarkably improve the ionic conductivity and broaden the oxidative stable potential (2.2 V vs Mg/Mg²⁺) on stainless steel. Reversible Mg electrochemical plating/stripping with a low overpotential below 200 mV and ca. 90% Coulombic efficiency are obtained. The current density of Mg plating/stripping is increased 238 times after the addition of PP₁₄TFSI, where the mechanism of competitive coordination of TFSI^- making an easier Mg plating/stripping is proposed theoretically. The MBA-2AlF₃ electrolyte with a ratio-optimized THF/PP₁₄TFSI cosolvent exhibits good compatibility with the Mo₆S₈ cathode. Furthermore, the Se@pPAN|Mg full cell exhibits an initial capacity of 447.8 mAh g^-1 and as low as ∼0.66% capacity decay per cycle for more than 70 cycles at 0.2 C with the synergy of LiTFSI additives. The facile modification strategy of ionic liquid in the MBA-based electrolyte sheds inspiring light on exploring non-nucleophilic and chlorine-free electrolytes for practical rechargeable magnesium batteries.

Designing Nanostructured Metal Chalcogenides as Cathode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are regarded as promising candidates for beyond-lithium-ion batteries owing to their high energy density. Moreover, as Mg metal is earth-abundant and has low propensity for dendritic growth, RMBs have the advantages of being more affordable and safer than the currently used lithium-ion batteries. However, the commercial viability of RMBs has been negatively impacted by slow diffusion kinetics in most cathode materials due to the high charge density and strongly polarizing nature of the Mg²⁺ ion. Nanostructuring of potential cathode materials such as metal chalcogenides offers an effective means of addressing these challenges by providing larger surface area and shorter migration routes. In this article, a review of recent research on the design of metal chalcogenide nanostructures for RMBs' cathode materials is provided. The different types and structures of metal chalcogenide cathodes are discussed, and the synthetic strategies through which nanostructuring of these materials can be achieved are described. An organized summary of their electrochemical performance is also presented, along with an analysis of the current challenges and future directions. Although particular focus is placed on RMBs, many of the nanostructuring concepts that are discussed here can be carried forward to other next-generation energy storage systems.

Progress and Perspective on Rechargeable Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur (Mg-S) batteries are emerging as a promising candidate for next-generation energy storage technologies owing to their prominent advantages in terms of high volumetric energy density, low cost, and enhanced safety. However, their practical implementation is facing great challenges in finding electrolytes that can fulfill a multitude of rigorous requirements along with efficient sulfur cathodes and magnesium anodes. This review highlights electrolyte design for reliable Mg-S batteries in terms of efficient Mg-based salt construction (cation/anion design of organomagnesium salt-based electrolytes, optimization of all inorganic salt-based electrolytes and choosing of simple salt-based electrolytes), suitable solvent selection, and strategies for confronting corrosivity of Mg electrolytes. Before the comprehensive overview of the research status of Mg-based electrolytes, the understanding of Mg-S electrochemistry and views on the recent progress and potential strategies for high-performance S-based cathode and Mg anode are also provided for a holistic insight into Mg-S systems. At the end, the perspectives on the possible research directions for constructing high performance practical Mg-S batteries are also shared.

Multi-ion Strategy toward Highly Durable Calcium/Sodium-Sulfur Hybrid Battery

Nonlithium (Li) metal-sulfur batteries are a viable technology for large-scale energy storage due to their relative high energy densities and low cost. However, their practical application is still hindered by the insufficient reversibility and/or limited cycling stability. Herein, we report a high-performance calcium/sodium-sulfur (Ca/Na-S) hybrid battery enabled by a multi-ion chemistry. The introduction of Na ions in the electrolyte greatly boosts the conversion of Ca polysulfides, which has been verified by theoretical calculation and experimental investigation. Meanwhile, the presence of Ca ions constructs a protective electrostatic shield around the initial protrusions on the Na metal anode without prereduction, thus efficiently suppressing the Na dendrite growth. The as-developed Ca/Na-S cell exhibited a high reversible capacity of 947 mAh g^-1 at 0.1 C with long cycle life, clearly demonstrating the feasibility of this multi-ion strategy for developing low-cost non-Li metal-sulfur batteries.

Performance Study of MXene/Carbon Nanotube Composites for Current Collector- and Binder-Free Mg-S Batteries

The realization of sustainable and cheap Mg-S batteries depends on significant improvements in cycling stability. Building on the immense research on cathode optimization from Li-S batteries, for the first time a beneficial role of MXenes for Mg-S batteries is reported. Through a facile, low-temperature vacuum-filtration technique, several novel current collector- and binder-free cathode films were developed, with either dipenthamethylene thiuram tetrasulfide (PMTT) or S8 nanoparticles as the source of redox-active sulfur. The importance of combining MXene with a high surface area co-host material, such as carbon nanotubes, was demonstrated. A positive effect of MXenes on the average voltage and reduced self-discharge was also discovered. Ascribed to the rich polar surface chemistry of Ti3 C2 Tx MXene, an almost doubling of the discharge capacity (530 vs. 290 mA h g-1 ) was achieved by using MXene as a polysulfide-confining interlayer, obtaining a capacity retention of 83 % after 25 cycles.

Material design strategies to improve the performance of rechargeable magnesium-sulfur batteries

Beyond current lithium-ion technologies, magnesium-sulfur (Mg-S) batteries represent one of the most attractive battery chemistries that utilize low cost, sustainable, and high capacity materials. In addition to high gravimetric and volumetric energy densities, Mg-S batteries also enable safer operation due to the lower propensity for magnesium dendrite growth compared to lithium. However, the development of practical Mg-S batteries remains challenging. Major problems such as self-discharge, rapid capacity loss, magnesium anode passivation, and low sulfur cathode utilization still plague these batteries, necessitating advanced material design strategies for the cathode, anode, and electrolyte. This review critically appraises the latest research and design principles to address specific issues in state-of-the-art Mg-S batteries. In the process, we point out current limitations and open-ended questions, and propose future research directions for practical realization of Mg-S batteries and beyond.

Effect of Mg2+ and Mg2+/Li+ electrolytes on rechargeable magnesium batteries based on an erythrocyte-like CuS cathode

Rechargeable magnesium batteries (RMBs) have been regarded as one of the most promising competitors for energy storage systems owing to their high volumetric density (3800 W h cm-3), earth abundance and safe metallic Mg anode. However, up till now, only a few cathode materials and suitable electrolytes could be used for RMBs, which has hindered the applications for rechargeable magnesium batteries. In this study, erythrocyte-like CuS nanosheet assemblies were prepared via a hydrothermal process and applied to RMBs with Mg2+ and Mg2+/Li+ complex electrolytes. Erythrocyte-like CuS|Mg2+, Li+|Mg secondary batteries provide a stabilized capacity of 250 mA h g-1 without the activation process and excellent cycling stability over 500 cycles, which is superior to that in pure Mg2+ electrolytes. Further, the reaction mechanism and kinetic analysis revealed that the addition of lithium salts could not only enhance the electrochemical performance but also effectively avoid the activation process. Considering the outstanding electrochemical performance of CuS in the Mg2+/Li+ complex electrolyte, our study provides a new strategy for designing electrode structures and electrolyte composition for rechargeable magnesium batteries.

Interfaces in rechargeable magnesium batteries

This minireview provides a concise overview on the development of electrolytes for rechargeable magnesium (Mg) batteries. It elucidates the intrinsic driving force of the evolution from Grignard-based electrolytes to electrolytes based on simple Mg salts. Additional discussion includes the key electrochemical processes at the interfaces in Mg electrolytes, with a focus on unaddressed issues and future research directions.

Mesoporous Nitrogen-Doped Carbon Nanospheres as Sulfur Matrix and a Novel Chelate-Modified Separator for High-Performance Room-Temperature Na-S Batteries

Room-temperature sodium-sulfur (RT/Na-S) batteries are considered among the most promising next-generation energy storage and conversion systems because of the earth-abundant reserves of sodium and sulfur. These batteries also possess the advantages of high theoretical gravimetric capacity, high energy density, and low cost. Herein, highly uniform Fe³⁺ /polyacrylamide nanospheres (FPNs) are fabricated on a large-scale by a facile, low-cost approach. Subsequently, mesoporous nitrogen-doped carbon nanospheres (PNC-Ns), obtained by carbonizing FPNs, are applied as a sulfur matrix to improve the utilization of sulfur, enhance the overall conductivity of the cathode, and inhibit the shuttling of sodium polysulfides (SPSs). In addition, graphene and FPNs are simultaneously coated onto the side of the separator to form a FPNs-graphene-functionalized separator (FPNs-G/separator); here, the mesoporous FPNs effectively anchor and block the SPSs, while the large specific area graphene sheets eliminate the intrinsic mechanical brittleness of the FPNs and improve the overall conductivity of RT/Na-S batteries. When S/PNC-Ns as a cathode and FPNs-G/separator are assembled into an RT/Na-S battery, it delivers a high discharge capacity (639 mAh g^-1 at 0.1 C after 400 cycles), stable cycle life (396 mAh g^-1 at 0.5 C after 800 cycles), and good rate performance (228 mAh g^-1 at 2 C).

Approaching the voltage and energy density limits of potassium-selenium battery chemistry in a concentrated ether-based electrolyte

Potassium–selenium (K–Se) batteries offer fairly high theoretical voltage (∼1.88 V) and energy density (∼1275 W h kg_Se⁻¹). However, in practice, their operation voltage is so far limited to ∼1.4 V, resulting in insufficient energy utilization and mechanistic understanding. Here, it is demonstrated for the first time that K–Se batteries operating in concentrated ether-based electrolytes follow distinctive reaction pathways involving reversible stepwise conversion reactions from Se to K₂Se_x (x = 5, 3, 2, 1). The presence of redox intermediates K₂Se₅ at ∼2.3 V and K₂Se₃ at ∼2.1 V, in contrast with previous reports, enables record-high average discharge plateau voltage (1.85 V) and energy density (998 W h kg_Se⁻¹ or 502 W h kg_K2Se⁻¹), both approaching the theoretical limits and surpassing those of previously reported Na/K/Al–Se batteries. Moreover, experimental analysis and first-principles calculations reveal that the effective suppression of detrimental polyselenide dissolution/shuttling in concentrated electrolytes, together with high electron conductibility of Se/K₂Se_x, enables fast reaction kinetics, efficient utilization of Se, and long-term cyclability of up to 350 cycles, which are impracticable in either K–S counterparts or K–Se batteries with low/moderate-concentration electrolytes. This work may pave the way for mechanistic understanding and full energy utilization of K–Se battery chemistry.

K–Se batteries follow distinctive reaction pathways in concentrated ether electrolytes, and deliver record-high discharge plateau voltage of 1.85 V on average and energy density of 998 W h kg_Se⁻¹, both approaching the theoretical limits.

A highly-effective nitrogen-doped porous carbon sponge electrode for advanced K–Se batteries

A rechargeable K–Se battery is emerging as an energy storage system because of its much higher specific capacity than that of the traditional alkali metal-ion batteries, but is facing some critical issues and challenges.

Enabling Mg metal anodes rechargeable in conventional electrolytes by fast ionic transport interphase

Rechargeable magnesium batteries have received extensive attention as the Mg anodes possess twice the volumetric capacity of their lithium counterparts and are dendrite-free. However, Mg anodes suffer from surface passivation film in most glyme-based conventional electrolytes, leading to irreversible plating/stripping behavior of Mg. Here we report a facile and safe method to obtain a modified Mg metal anode with a Sn-based artificial layer via ion-exchange and alloying reactions. In the artificial coating layer, Mg₂Sn alloy composites offer a channel for fast ion transport and insulating MgCl₂/SnCl₂ bestows the necessary potential gradient to prevent deposition on the surface. Significant improved ion conductivity of the solid electrolyte interfaces and decreased overpotential of Mg symmetric cells in Mg(TFSI)₂/DME electrolyte are obtained. The coated Mg anodes can sustain a stable plating/stripping process over 4000 cycles at a high current density of 6 mA cm⁻². This finding provides an avenue to facilitate fast ion diffusion kinetics of Mg metal anodes in conventional electrolytes.

Rechargeable Mg-M (M = Li, Na and K) dual-metal-ion batteries based on a Berlin green cathode and a metallic Mg anode

Mg-M (M = Li, Na and K) dual-metal-ion batteries featuring a dendrite-free Mg anode and an alkali-metal-ion storage cathode are promising safe energy storage systems. However, the compatibility between cathode materials and insertion cations might largely limit the electrochemical performance of the cathodes. In this work, three types of Mg-M (M = Li, Na and K) dual-metal-ion batteries are constructed with a Berlin green (FeFe(CN)6) cathode. The FeFe(CN)6 cathode is compatible with the dual-salt Mg2+/M+ (M = Li, Na and K) electrolytes, and delivers a high reversible capacity of 120 mA h g-1 at 50 mA g-1, with no capacity fading over 50 cycles in Mg-Na batteries. The Mg-Na battery also shows an outstanding rate capability, providing 85 mA h g-1 at 1000 mA g-1 and superior long-term cyclability over 800 cycles. The electrochemical performance comparison between Mg-Li, Mg-Na and Mg-K dual-metal-ion batteries demonstrates the significance of the appropriate hydrated ionic radius and dehydrated ionic radius for the insertion of cations with the FeFe(CN)6 cathode. This work provides new design strategies for stable and high energy density cathodes, and opens a new avenue for building safe and high-performance Mg-M (M = Li, Na and K) dual-metal-ion batteries for practical applications.

Current Status and Future Prospects of Metal-Sulfur Batteries

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

The Rechargeable Aluminum Battery: Opportunities and Challenges

Aluminum battery systems are considered as a system that could supplement current lithium batteries due to the low cost and high volumetric capacity of aluminum metal, and the high safety of the whole battery system. However, first the use of ionic liquid electrolytes leading to AlCl₄ ^- instead of Al³⁺ , the different intercalation reagents, the sluggish solid diffusion process and the fast capacity fading during cycling in aluminum batteries all need to be thoroughly explored. To provide a good understanding of the opportunities and challenges of the newly emerging aluminum batteries, this Review discusses the reaction mechanisms and the difficulties caused by the trivalent reaction medium in electrolytes, electrodes, and electrode-electrolyte interfaces. It is hoped that the Review will stimulate scientists and engineers to develop more reliable aluminum batteries.

In Situ X-ray Absorption Spectroscopic Investigation of the Capacity Degradation Mechanism in Mg/S Batteries

The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge-discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS)₂-AlCl₃ electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg₃S₈, through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS₂ as a catalyst to activate the irreversible reaction of low-order MgS _x and MgS, which results in an increased discharging capacity up to 900 mAh·g^-1 and a longer cycling life.

High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes for Magnesium-Sulfur Batteries

The shortage of high-performance and easily prepared electrolyte has hindered the progress of rechargeable magnesium-sulfur (Mg-S) batteries. In this paper, we develop a new electrolyte based on Mg(CF₃SO₃)₂-AlCl₃ dissolved in tetrahydrofuran and tetraglyme mixed solvents. Mg(SO₃CF₃)₂ as an Mg²⁺ source is nonnucleophilic, easy to handle, and much cheaper than Mg(TFSI)₂ (TFSI = bis(trifluoromethanesulfonyl)imide). After modification with anthracene (π stabilizing agent) as a coordinating ligand to stabilize the Mg²⁺ ions and MgCl₂ to improve the interface properties by accelerating the reaction of Mg(CF₃SO₃)₂ with AlCl₃, the electrolyte exhibits a low overpotential for overall Mg deposition and dissolution, moderate anodic stability (3.25 V on Pt, 2.5 V on SS, 2.0 V on Cu, and 1.85 V on Al, respectively), and a suitable ionic conductivity (1.88 mS cm^-1). More importantly, this electrolyte modulated by Li-salt additives exhibits good compatibility with S cathode and can be applicable for Mg-S batteries. The rational formulation of the new electrolyte could provide a new avenue for simply prepared Mg electrolytes of Mg-S and rechargeable magnesium batteries.

Stacking of Tailored Chalcogenide Nanosheets around MoO2-C Conductive Stakes Modulated by a Hybrid POM⊂MOF Precursor Template: Composite Conversion-Insertion Cathodes for Rechargeable Mg-Li Dual-Salt Batteries

Mg anode has pronounced advantages in terms of high volumetric capacity, resource abundance, and dendrite-free electrochemical plating, which make rechargeable Mg-based batteries stand out as a representative next-generation energy storage system utilized in the field of large-scale stationary electric grid. However, sluggish Mg²⁺ diffusion in cathode lattices and facile passivation on the Mg anode hinder the commercialization of Mg batteries. Exploring a highly electroactive cathode prototype with hierarchical nanostructure and compatible electrolyte system with the capability of activating both an anode and a cathode is still a challenge. Here, we propose a POM⊂MOF (NENU-5) core-shell architecture as a hybrid precursor template to achieve the stacking of tailored chalcogenide nanosheets around MoO₂-C conductive stakes, which can be employed as conversion-insertion cathodes (Cu_1.96S-MoS₂-MoO₂ and Cu₂Se-MoO₂) for Mg-Li dual-salt batteries. Li-salt modulation further activates the capacity and rate performance at the cathode side by preferential Li-driven displacement reaction in Cu⁺ extrusible lattices. The heterogeneous conductive network and conformal dual-doped carbon coating enable a reversible capacity as high as 200 mAh/g with a coulombic efficiency close to 100%. The composite cathode can endure a long-term cycling up to 400 cycles and a high current density up to 2 A/g. The diversity of MOF-based materials infused by functional molecules or clusters would enrich the nanoengineering of electrodes to meet the performance demand for future multivalent batteries.

Nonlithium Metal-Sulfur Batteries: Steps Toward a Leap

Present mobile devices, transportation tools, and renewable energy technologies are more dependent on newly developed battery chemistries than ever before. Intrinsic properties, such as safety, high energy density, and cheapness, are the main objectives of rechargeable batteries that have driven their overall technological progress over the past several decades. Unfortunately, it is extremely hard to achieve all these merits simultaneously at present. Alternatively, exploration of the most suitable batteries to meet the specific requirements of an individual application tends to be a more reasonable and easier choice now and in the near future. Based on this concept, here, a range of promising alternatives to lithium-sulfur batteries that are constructed with non-Li metal anodes (e.g., Na, K, Mg, Ca, and Al) and sulfur cathodes are discussed. The systems governed by these new chemistries offer high versatility in meeting the specific requirements of various applications, which is directly linked with the broad choice in battery chemistries, materials, and systems. Herein, the operating principles, materials, and remaining issues for each targeted battery characteristics are comprehensively reviewed. By doing so, it is hoped that their design strategies are illustrated and light is shed on the future exploration of new metal-sulfur batteries and advanced materials.